The present invention relates in general to ballistic pressure transducers or gauges, and in particular to a new and useful method and apparatus for the dynamic calibration of ballistic pressure transducers. According to the invention, large positive pressure pulses are generated which have very short rise times and which can be utilized for the dynamic calibration of ballistic pressure transducers or gauges. Pressure pulses are generated which are up to 150,000 lbs. per square inch with rise times of less than 600 microseconds.
Ballistic pressure gauges are routinely calibrated statically against deadweight pressure standards to obtain pressure vs. static response characteristics. The strength of this calibration technique is its traceability to primary pressure standards. The weakness of this technique however, is in the assumption that the static and dynamic responses of the gauge are identical. Any differences in gauge response between static (calibration) and dynamic (measurement) events will not appear utilizing this calibration technique. Dynamic calibration techniques are necessary to overcome this problem. The generation of precisely known high pressure pulses, however, is not a simple matter. Three general dynamic calibration techniques have been utilized.
The first of these is a negative going pressure step or pulse method. In this technique the gauge is exposed to a given pressure under static conditions using a hydraulic fluid. The gauge is then sealed off from the hydraulic system and its output is brought to zero. The pressure on the gauge is then relieved using a fast acting dump valve to bring the system to atmospheric pressure. The gauge output obtained during the depressurization is assumed to be the inverse of the corresponding positive pressure pulse or step.
Strengths of this technique include its relative simplicity and suitability to use in calibration facilities. The response of the negative step calibrator can be very quick, i.e. 100 microseconds or less. The major assumption, however, that the pressure response of the gauge is equal and opposite to the negative response of the gauge, is not completely accurate. Pressure preloading of the gauge to mount interface and hysteresis causes significant differences between the responses to pressurization and depressurization pulses.
Another technique utilizes a ballistic pulse. In this technique the gauge is mounted at the end of the tube, in contact with a hydraulic fluid confined by a movable piston. The tube guides a projectile which impacts the piston to create a positive pulse in the fluid. Different pressures may be achieved by varying the compressibility of the fluid, the mass of the piston, and the mass and velocity of the projectile. The pulses rise within milliseconds and mimic the characteristic rising and falling of a ballistic pressure pulse.
The ballistic pressure method is quite useful for dynamic comparison of several different pressure gauges. Variations in projectile velocity, frictional effects on the moving piston, and other energy losses make it difficult to accurately compute the actual delivered pressures. Because the projectile is fired during the calibration process, this method requires more extensive safety provisions than are readily available in most laboratories.
The final dynamic calibration technique utilizes a shock tube. Two general approaches are followed. In the first, the test gauge is mounted in the end wall of a closed tube and a shock wave is generated from the opposite end of the tube. The gauge output is monitored as the shock front arrives and stagnates at the end wall. In the second approach, the gauge is mounted in the side wall of the tube and its output is monitored as the shock front passes. Both methods generate rapidly rising pressure pulses that are readily calibrated using temperature and velocity measurements and gas properties.
Shock tube methods are being successfully used to establish the dynamic response characteristics of pressure gauges. Calibration however, is generally limited to pressures below 1,000 lbs. per square inch, whereas commercial applications required calibration up to 25,000 per square inch and defense applications up to 150,000 lbs. per square inch. Shock tubes also pose an acoustical hazard.